PSI - Issue 3

3

Laura D’Agostino et al. / Procedia Structural Integrity 3 (2017) 201–207 Author name / Structural Integrity Procedia 00 (2017) 000–000

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 during the tensile test, the tensile holder (Fig1a) was located in the testing machine (Fig. 1b) and the load was applied using the screw rotated by a step-motor;  the tensile test was stopped corresponding to different values of the macroscopic deformation and the tensile holder with the stresse specimen was located in the SEM vacuum chamber.  Lateral surfaces were observed step by step by SEM focusing on the damaging micromechanisms in graphite nodules due to tensile tests.

Table 1. Investigated fully ferritic DCI chemical composition.

C

Si

Mn

S

P

Cu

Cr

Mg

Sn

3.62

2.72

0.19

0.011

0.021

0.019

0.031

0.047

0.011

Table 2. Investigated ferritized DCI chemical composition.

C

Si

Mn

S

P

Cu

Cr

Mg

Ni

3.32

2.36

0.31

0.012

0.016

0.62

0.058

0.033

0.025

Applied loads and specimens deformations were measured using, respectively, two miniature load cells (10 kN each) and a Linear Variable Differential Transformer (LVDT).

(a) (b) Fig. 1. Tensile holder with mini tensile specimen (a) and tensile test machine (b).

3. Experimental results and discussion Considering the as cast ferritic DCI, Di Cocco (2014), the main damaging micromechanism is the so called “onion like” mechanism (Figs 2-4). This mechanism, Di Cocco (2013), can be described as an internal debonding between:  a nodule core, obtained directly from the melt during the solidification process and characterized by lower nanohardness values and lower wearing resistance,  a nodule shield, obtained during the cooling stage and due to a carbon solid diffusion through the austenitic shield obtained during the eutectic reaction (characterized by higher nanohardness values and a higher wearing resistance). In addition, a secondary damage mechanism is often observed (Fig. 2 and Fig. 3). After the ‘‘onion-like” mechanism initiation, some secondary cracks could initiate near the nodule centre and propagate with the stress increase (“disgregation” mechanism). Focusing on the ferritic matrix, the increase of the applied load implies an increase of the presence of slip bands, usually but not uniquely in proximity to the nodule equators, with the initiation and propagation of secondary cracks (Figs 2-4). Secondary cracks propagation and “coalescence” implies the final collapse of the specimen corresponding to the UTS value.